functions.php

<?php

/**
 * Remove Image Alignment from Featured Image
 *
 */
function be_remove_image_alignment( $attributes ) {
  $attributes['class'] = str_replace( 'alignleft', 'alignnone', $attributes['class'] );
	return $attributes;
}
add_filter( 'genesis_attr_entry-image', 'be_remove_image_alignment' );